Tinnitus is the most common work related disabilities of veterans returning from Iraq and Afghanistan. More than 1.5 million former service members report a ringing in the ears when no noise is actually present. One out of every two combat veterans, report having this sometimes debilitating condition without a cure.
The issue of Tinnitus is largely a mystery, as it is a phantom sound heard in the absence of actual sound. Tinnitus patients “hear” ringing, buzzing or hissing in their ears such as how an amputee might “feel” pain in a missing limb. It is a symptom, not a disease, and though it can be caused by exposure to loud noise in some cases, there is no apparent trigger. Existing treatments are unreliable, and will either not working at all or will vary greatly in effectiveness for the few that report some relief.
“But a global research effort involving investigators from the University at Buffalo; Southeast University in Nanjing, China; and Dalhousie University in Nova Scotia, Canada, have made a major breakthrough that provides new insights into how tinnitus, and the often co-occurring hyperacusis, a condition that causes sounds to be perceived as intolerably loud, might develop and be sustained.”
The results of this study, to be published in eLife, have suggested that the neural network responsible is more expansive than had been previously thought. These findings could lead to a testable model that helps to identify which region or regions of the brain might be responsible for causing the two conditions. The aim is to eventually test the model by deactivating specific segments of the neural network once they have conceptualized a broader, more comprehensive understanding of the neural network. By process of elimination, the hope is to learn if shutting down one part of that network relieves tinnitus and or hyperacusis.
“Until the mid-1990s, tinnitus was thought to be centered in the ear, but patients who lost their hearing on one side after a surgical tumor removal unrelated to the condition reported still hearing a ringing — in their deaf ear. “This changed the thinking in the field,” says Richard Salvi, director of UB’s Center for Hearing and Deafness, and one of the study’s authors. “Having severed the neural connection between the ear and the brain, it’s impossible for the phantom sound to be generated in the ear. It has to be generated in the brain.”
Even though exactly where and how tinnitus occurs in the brain is currently unknown, Salvi says that their functional MRI studies show the abnormal activity underlying tinnitus and hyperacusis isn’t confined to a specific brain location, but actually involves a neural network. Unlike traditional MRIs, which only show neural structure, functional MRIs show what parts of the brain id active at a point in time. Functional connectivity MRI is also able to reveal how one part of the brain interacts with other regions.
The researchers induced tinnitus in rats by administering the active ingredient in aspirin, which has long been known to produce tinnitus and hyperacusis symptoms in humans. “Certain brain regions become very active once tinnitus is induced, much more so than it is for an animal with normal hearing,” says Salvi. “Even though high-dose aspirin induces a hearing loss and less information is being sent from the ear to the brain as a result, the brain responds with greater activity. It’s paradoxical, like a car getting better gas mileage with a less efficient engine.”
Tracing the network’s course, researchers identified a major component of the central auditory pathway, the sound processing center of the brain. Although other research has shown this activity, the research currently being done is novel due to the appearance of the amygdala. This is the part of the brain responsible for assigning emotion to our perceptions. Many patients had reported the onset of tinnitus after experiencing significant stress or anxiety and therefore leading researchers to believe that the condition may not just be related to hearing loss. Instead, there are other emotional factors working together with the auditory factors.
“The reticular formation, an arousal center involved in the “fight or flight” response is active too, plus the hippocampus, the memory region of the brain that helps identify where things are located, such as the location of the phantom sound. So the auditory system is connecting sound to a location, the ear in this case.”
There is emotion and arousal, and an interesting final piece to the network is activity of the cerebellum, responsible for motor planning such as reaching for a cup or catching a ball.
“We were shocked when this part of the brain popped up,” says Salvi. “Almost all parts of the network can be explained: location of sound; the emotional attachment; why people get aroused when they have tinnitus; we’re puzzled by the cerebellum involvement, but it might act like some kind of “gate” that’s allowing the phantom sound to enter the consciousness,” says Salvi.”
